A new study of Canadian populations has found that people living in rural households are less likely to be diagnosed with inflammatory bowel disease than individuals living in urban households, particularly when the disease starts in childhood.
The research also shows that spending the first 5 years of life in a rural household is tied to a lower risk of developing inflammatory bowel disease (IBD) later in life.
The study, which is published in the The American Journal of Gastroenterology, is the work of teams from the Children's Hospital of Eastern Ontario (CHEO) Research Institute in Ottawa and several other research centres in Canada.
IBD is a group of diseases wherein the gut, or gastrointestinal (GI) tract, becomes damaged as a result of chronic inflammation.
For example, Crohn's disease can affect any part of the GI tract between the mouth and the anus, whereas ulcerative colitis only affects the colon, or large intestine, and the rectum. Also, in Crohn's disease, inflammation can occur in all layers of the gut, whereas in ulcerative colitis it only occurs in the innermost layer of the colon.
IBD rising worldwide
Scientists do not know exactly what causes IBD, but they suspect that it arises from a faulty immune reaction that mistakenly attacks the body's own tissue to cause inflammation. They suggest that such a reaction could be triggered by environmental factors that some people, because of their genetic makeup, are more susceptible to.
In the United States, estimates suggest that around
In Canada, where the new study was conducted, there are approximately 233,000 people living with IBD. Canada has one of the highest incidences of IBD in the world, and the rates are continuing to rise - especially among very young children, note the authors.
In their study paper, the researchers explain that rates of IBD have also been rising in other parts of the world over the past 50 years. They suggest that increased urbanization could be a reason, perhaps due to its influence on the composition of microbes in the gut, particularly in early childhood.
Need to study IBD and environment
The researchers note that while an analysis of dozens of studies has pointed to a link between increased urbanization and higher incidence of IBD, the results are full of inconsistencies, which could partly be due to the many different definitions of rurality.
They also point out that there has been no assessment of the link between timing of urban exposure and age at which IBD is diagnosed.
They argue, therefore, that it could be very useful for IBD research to investigate in more detail the link between age at which IBD is diagnosed and the age of exposure to a rural or urban environment.
Clear results from such an investigation could, for example, help researchers to pinpoint the interactions between genetics, environment, and gut microbes that lead to IBD. This could help them to establish the age at which intervention could best prevent the disease.
More IBD in urban population
Thus, the team decided to carry out a population-based study, using validated data from official health records, to assess the risk for IBD in people living in Canada, based on rural or urban residence.
First, they analyzed the link between rural or urban dwelling and age at diagnosis of IBD, and then they assessed the extent to which continuing to live in a rural or urban environment after birth raised the risk of developing IBD later in life.
The researchers identified a total of 45,567 people diagnosed with IBD who were living in four provinces in Canada (Alberta, Manitoba, Nova Scotia, and Ontario) between 1999 and 2010. Of these, 6,662 lived in rural households and 38,905 lived in urban households over that period.
Based on those figures, they calculated the incidence of IBD to be 30.72 per 100,000 person-years in the rural population and 33.16 per 100,000 in the urban population.
'Children more strongly affected'
Further analysis revealed that children appear to be at lower risk of developing IBD if they live in a rural household - particularly before the age of 10.
"This effect is particularly strong in children who are raised in a rural household in the first 5 years of life," explains lead author Dr. Eric Benchimol, a pediatric gastroenterologist at the CHEO Inflammatory Bowel Disease Centre.
Dr. Benchimol, also of the Institute for Clinical Evaluative Sciences in Toronto, Canada, argues that the findings are particularly important because in previous work, they discovered that there has been a significant rise in the number of very young children diagnosed with IBD in the past 20 years.
He suggests that the study increases our understanding of how environmental factors that make people more susceptible to IBD may affect children more strongly than adults.
"We've known that in addition to genetic risk factors, environmental factors have been associated with the risk of developing IBD. But this new study demonstrates the importance of early life exposure in altering the risk of IBD, and that needs further study."
Dr. Eric Benchimol